In a continuous casting steel-making process, molten steel is poured from a ladle into a large holding vessel known as a tundish. The tundish has one or more outlets through which the molten steel flows into one or more respective moulds. The molten steel cools and begins to solidify in the moulds to form continuously cast solid lengths of metal. A submerged entry nozzle is located between each tundish outlet and each mould, and guides molten steel flowing through it from the tundish to the mould. A stopper rod is often employed to control the flow rate of the molten steel through the submerged entry nozzle.
The stopper rod generally comprises an elongate body having a rounded nose at one end thereof. In use, the rod is orientated vertically along its axis and is disposed with its nose adjacent the throat of the submerged entry nozzle such that raising and lowering of the stopper rod opens and closes the inlet of the submerged entry nozzle and thereby controls the flow of metal therethrough. The nose of the stopper rod is sized to completely close the inlet of the submerged entry nozzle when lowered to a seated position within the throat of the submerged entry nozzle.
A particular problem associated with the casting of molten metal is that inclusions (e.g. alumina) are often present in the molten metal as it is flowed from the tundish to the mould. Such inclusions tend to deposit on the stopper rod nose or within the submerged entry nozzle depending upon the flow conditions within the casting channel. Accordingly, over time the build up of inclusions can affect the geometry of the components to such an extent that the flow control characteristics of the system are altered and the continuous casting sequence may have to be interrupted.
The injection of a stream of inert gas, such as argon, into the flow of molten metal helps to float out inclusions (especially alumina) and inhibits the build up of such inclusions, which can result in the clogging of parts of the system. The gas may be introduced in a number of ways, for example, through the central axis of a stopper rod to exit through a discharge port in the nose of the stopper rod, or through a porous plug located in the base of a tundish or in the bore of a submerged entry nozzle. Often, due to the venturi effect, molten metal flowing past the point of introduction of the gas creates a negative pressure which can be transmitted back into the gas supply, potentially sucking air into the metal through the stopper rod or through the gas supply pipework if joints are not airtight. To date, this problem has been addressed by providing a restrictor in the gas supply channel, e.g. in the nose of the stopper rod. The restrictor may be provided by a simple narrowing of the bore or it may be constituted by a plug with a narrow bore therethrough (or a porous plug). The restrictor creates a backpressure and results in a positive internal pressure upstream of the restrictor. This positive internal pressure inhibits air ingress into the gas supply channel thereby reducing the quantity of contaminants in the metal being cast.
It will be understood that all references to pressure are relative to atmospheric pressure so that negative pressures relate to pressures below atmospheric pressure and positive pressures relate to pressures above atmospheric pressure.
A disadvantage of using a typical restrictor such as that described above, in a stopper rod, is that over time an increase in internal pressure can arise which can result in the stopper rod cracking or even being blown apart. Moreover, the required backpressure, coupled with a desired relatively low flow rate, dictates that the bore of the restrictor be small (e.g. 1 mm in diameter) and very accurately formed (i.e. with a low tolerance level). However, currently it is difficult to produce accurately dimensioned narrow bore refractories of such small diameters.
It is therefore an aim of the present invention to provide a flow control device that addresses the afore-mentioned problems.
It is also an aim of the present invention to provide a flow control device that can address similar problems in other systems where fluid is transported along a conduit.